Testing AI Focus

Hi

I have a 40D plus 17-85 IS USM and 70 300 IS USM.

In general I am very pleased with the pictures I get however I am not too sure of it when photographing moving objects ie motorsports, cycle riding etc as they are often either OOF or not sharp.

What I am looking for is a definitive guide on how to set the camera up for these situations, in particular grasstrack racing and BIF, as these are on the agenda for next weekend.

If anyone could post similar shots to compare with.

I want to give the 40D a really good test session to decide if it should go back or not for calibration and don't want to get poor shots through not trying a particular technique or setup.

Thanks.

I purchased a 40D a few months ago for use with motorsports / airshows, etc. - intending to use it as my primary body. To date the DSLRs I’ve used are the 10D, 350D, and 400D. After casual use of the 40D over the past few months (no motorsports events) I wasn’t as comfortable with the AI Servo performance as the 400D so I conducted the following test. (Note, I still have the 400D as a second body).

I had my wife drive toward me at a constant speed of 50 mph and I focused on the license plate (easiest point to test focus accuracy) for 4-reps and I tested both the 40D and 400D with the 400/5.6 lens.

I then had her drive 10mph and accelerate quickly to 50mph and did the same focus test.

My conclusions are: my 40D seems to be more “sensitive” than the 400D when I move off my target a little (it seems quicker to readjust focus). But when I keep on target the 40D came out a little sharper than the 400D…I suspect this greater “sensitivity” is what was bothering me…

I just finished an airshow this weekend and had both bodies – I’m very happy with the results of the 40D – and from casual inspection it was if anything better (certainly not worse) than my 400D. I was more careful in keeping the intended focal point on the object in the selected camera focal point at all times than I normally would and it seemd to perform well.
Brit

pkay2 said:

Hi

I have a 40D plus 17-85 IS USM and 70 300 IS USM.

In general I am very pleased with the pictures I get however I am not
too sure of it when photographing moving objects ie motorsports,
cycle riding etc as they are often either OOF or not sharp.

What I am looking for is a definitive guide on how to set the camera
up for these situations, in particular grasstrack racing and BIF, as
these are on the agenda for next weekend.

If anyone could post similar shots to compare with.

I want to give the 40D a really good test session to decide if it
should go back or not for calibration and don't want to get poor
shots through not trying a particular technique or setup.

Thanks.

Click to expand...

AI Focus starts in One-Shot, then switches to AI Servo if it thinks the subject has moved. This operation is imperfect, so much so that the professional EOS cameras have never included AI Focus.

For moving subjects use AI Servo.

Use AI Servo, enable all focus points.

In this mode, focus will be acquired by the center FP. As long as one FP is on the subject at all times, it will maintain focus.

I've only had my 40D for 2 weeks and that is how it works, for me.

--
I may be, and probably am, completely wrong.

All my comments were based on AI Servo, for some reason it didn't even occur to me that you may have been referring to AI Focus mode...but as clearly stated in your subject line that is your question...and as others have already stated use AIServo...gotta read more carefully...

Brit said:

pkay2 said:

Hi

I have a 40D plus 17-85 IS USM and 70 300 IS USM.

In general I am very pleased with the pictures I get however I am not
too sure of it when photographing moving objects ie motorsports,
cycle riding etc as they are often either OOF or not sharp.

What I am looking for is a definitive guide on how to set the camera
up for these situations, in particular grasstrack racing and BIF, as
these are on the agenda for next weekend.

If anyone could post similar shots to compare with.

I want to give the 40D a really good test session to decide if it
should go back or not for calibration and don't want to get poor
shots through not trying a particular technique or setup.

Thanks.

Click to expand...

Click to expand...

Sorry, I must apologise, my original post was done rather quickley as I was at work, AI Servo is what I have been using.

I have seen previous posts about using the custom functions to change the way this works but is this really required and if so what functions are set.

Sorry for the confusion

I use AI Servo for motorsports with my 40D and it is almost spot on. In fact, I shot some in low light/rain over the weekend and I was very pleased with the 40D.

Also keep in mind that AI Servo is only as fast as the AF speed of the lenses you are using. Not all lenses are created equal in my opinion....even USM lenses. I have found that my 200L and 400L primes are faster to AF than their L series zoom counterparts. Also the AF speed on the L lenses are faster than the "EF-S" and prosumer grade lenses.

I once shot a car coming head on with the 28-75 Tamron lens. I would say that 98 out of 100 were OOF. The AF speed on the Tamron is dog slow and could not come close to keeping up.

--
'Image is everything'
http://www.terrywatsonphoto.com
http://www.watsonfx.com

Without resorting to the custom functions there is choice between selecting a single focus sensor, or using Auto AF Point Selection. In AI Servo, the auto point selection always starts with the center sensor and I prefer to use it that way.

The custom function choices are between activating AF with a half-press of the shutter button, the AF-ON button (C.Fn IV-1 = 2 or 3), or the * button (C.Fn IV-2 = 1). I normally leave AF activation with the shutter button, but there can be times when you want AF and exposure separated.

pkay2 said:

Sorry, I must apologise, my original post was done rather quickley as
I was at work, AI Servo is what I have been using.

I have seen previous posts about using the custom functions to change
the way this works but is this really required and if so what
functions are set.

Sorry for the confusion

Click to expand...

Thanks for the ideas so far, what shutter speeds etc would you suggest for BIF for which I would use the 70-300 ?

Well, I mean for most BIF shots, I want the fastest shutter speed possible. Obviously there is a balance with how much depth of field and ISO noise you are willing to sacrifice. Also keep in mind that many lenses aren't their best wide open. I usually try to get a shutter speed of 1/800 or better if possible.

Here are a few of my BIFs









--
'Image is everything'
http://www.terrywatsonphoto.com
http://www.watsonfx.com

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